Support leading vertical bar in union types (issue #465)

[juanscasado]

This PR adds support for an optional leading vertical bar (|) in union type definitions in SDL, as specified in the official GraphQL specification.

Highlights:

The SDL parser now accepts union definitions with or without a leading vertical bar.
Added tests to ensure correct parsing of both formats.
Updated documentation (README) to reflect this new feature.
This change improves compatibility with other GraphQL tools and makes it easier to format long union type lists.

Reference: GraphQL Spec - Unions

Closes #465.

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[hlship]

I'm finally getting a chance to look into the backlog of PRs for lacinia --- can you sign the CLA?

[juanscasado]

Hi! Yes, I’ve already signed the CLA.
Thank you for giving me the opportunity to collaborate!

[hlship]

I was surprised this worked w/o changes to the grammar, but you can see in the grammar that a leading | is expressly allowed.